Theoretical Framework and Guidelines for the Cyclic Voltammetry of Closed Bipolar Cells

Closed bipolar cells (cBPCs) can offer valuable platforms for the development of electrochemical sensors. On the other hand, such systems are more intricate to model and interpret than conventional systems with a single polarizable interface, with the applied potential "splitting" into two polarized interfaces where two coupled charge transfers take place concomitantly. As a result, the voltammetry of cBPCs shows peculiarities that can be misleading if analyzed under the framework of classic electrochemical cells. In this work, rigorous mathematical solutions are deduced for the cyclic voltammetry (CV) of cBPCs, including the current-potential response, the interfacial potentials, and the interfacial redox concentrations. With such theoretical tools, a comprehensive view of the behavior of cBPCs can be gained, and adequate diagnosis criteria are established on the basis of the shape, magnitude, and position of the CV signal as a function of the scan rate and of the experimental conditions in the anodic and cathodic compartments.

Fluorination Effect on the Gibbs Transfer Energy for Methylene Group from 1,2-Dichloroethane or 1,1,1,2,3,4,4,5,5,5-​Decafluoropentane to Water

The ion-transfer reactions of alkyl and perfluoroalkyl carboxylate ions (CH₃(CH₂)_n-2COO^- and CF₃(CF₂)_n-2COO^-) at 1,2-dichloroethane (DCE) | water (W) and 1,1,1,2,3,4,4,5,5,5-decafluoropentane (DFP) | W interfaces were investigated. These ions gave reversible or quasi-reversible voltammetric waves due to their ion transfer across the interfaces, and the formal potentials and the formal Gibbs transfer energies from a non-aqueous solvent to water, ΔG°^'_tr,α→W (α = DCE and DFP), were determined. The ΔG°^'_tr,α→W for CH₃(CH₂)_n-2COO^- and CF₃(CF₂)_n-2COO^- linearly increased with n, allowing for estimating ΔG°^'_tr,α→W for methylene groups. The estimated value of ΔG°^'_tr,DCE→W for the -CH₂- group was higher than that of ΔG°^'_tr,DCE→W for the -CH₂- group, whereas the ΔG°^'_tr,DCE→W for the -CF₂- group was lower than that of ΔG°^'_tr,DCE→W for the -CF₂- group, indicating that the -CH₂- (or -CF₂-) group is more favorably (or unfavorably) solvated in the DCE phase compared to the DFP phase. From the estimated values, the fluorination effect of alkyl chains on partitioning the alkyl group between the biphase media has also been discussed.

Ion-Transfer Voltammetry at Fluorous Ether | Water Interfaces

This paper describes that fluorous ethers, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, can be used for a non-aqueous medium in electrochemistry at a liquid | liquid interface. These solvents dissolved a high concentration of tetraalkylammonium salts with highly-fluorinated anions, such as bis(nonafluorobutanesulfonyl)imide and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ions, and the solution had a high conductivity. The fluorous ether | water interfaces exhibited a substantial polarizable potential window, and various ions, including tetraphenylarsonium and tetraphenylborate ions, gave a reversible voltammetric wave due to their ion transfer across the interfaces. Using the tetraphenylarsonium-tetraphenylborate assumption, the formal potentials for the ion transfer and the formal Gibbs energies of ion transfer from the ethers to water were estimated. The Gibbs energies were close to those from a previously reported fluorous solvent, 1,1,1,2,3,4,4,5,5,5-decafluoropentane; the fluorous ethers also exhibited a higher affinity for fluorinated ions. Because of a lower volatility, the fluorous ethers would be more advantageously used in two-phase electrochemistry, particularly concerning analytical purposes.

Double Transfer Voltammetry in Two-Polarizable Interface Systems: Effects of the Lipophilicity and Charge of the Target and Compensating Ions

Analytical expressions are obtained for the study of the net current and individual fluxes across macro- and micro-liquid/liquid interfaces in series as those found in ion sensing with solvent polymeric membranes and in ion-transfer batteries. The mathematical solutions deduced are applicable to any voltammetric technique, independently of the lipophilicity and charge number of the target and compensating ions. When supporting electrolytes of semihydrophilic ions are employed, the so-called double transfer voltammograms have a tendency to merge into a single signal, which complicates notably the modeling and analysis of the electrochemical response. The present theoretical results point out that the appearance of one or two voltammetric waves is highly dependent on the size of the interfaces and on the viscosity of the organic solution. Hence, the two latter can be adjusted experimentally in order to "split" the voltammograms and extract information about the ions involved. This has been illustrated in this work with the experimental study in water | 1,2-dichloroethane | water cells of the transfer of the monovalent tetraethylammonium cation compensated by anions of different lipophilicity, and also of the divalent hexachloroplatinate anion.

Theoretical Treatment of Ion Transfers in Two Polarizable Interface Systems When the Analyte Has Access to Both Interfaces

A new theory is presented to tackle the study of transfer processes of hydrophilic ions in two polarizable interface systems when the analyte is initially present in both aqueous phases. The treatment is applied to macrointerfaces (linear diffusion) and microholes (highly convergent diffusion), obtaining analytical equations for the current response in any voltammetric technique. The novel equations predict two signals in the current-potential curves that are symmetric when the compositions of the aqueous phases are identical while asymmetries appear otherwise. The theoretical results show good agreement with the experimental behavior of the "double transfer voltammograms" reported by Dryfe et al. in cyclic voltammetry (CV) ( Anal. Chem. 2014 , 86 , 435 - 442 ) as well as with cyclic square wave voltammetry (cSWV) experiments performed in the current work. The theoretical treatment is also extended to the situation where the target ion is lipophilic and initially present in the organic phase. The theory predicts an opposite effect of the lipophilicity of the ion on the shape of the voltammograms, which is validated experimentally via both CV and cSWV. For the above two cases, simple and manageable expressions and diagnosis criteria are derived for the qualitative and quantitative study of ion lipophilicity. The ion-transfer potentials can be easily quantified from the separation between the two signals making use of explicit analytical equations.

Characterization of inclusion complexes of organic ions with hydrophilic hosts by ion transfer voltammetry with solvent polymeric membranes

The quantitative characterization of inclusion complexes formed in aqueous phase between organic ions and hydrophilic hosts by ion-transfer voltammetry with solvent polymeric membrane ion sensors is studied, both in a theoretical and experimental way. Simple analytical solutions are presented for the determination of the binding constant of the complex from the variation with the host concentration of the electrochemical signal. These solutions are valid for any voltammetric technique and for solvent polymeric membrane ion sensors comprising one polarisable interface (1PI) and also, for the first time, two polarisable interfaces (2PIs). Suitable experimental conditions and data analysis procedures are discussed and applied to the study of the interactions of a common ionic liquid cation (1-octyl-3-metyl-imidazolium) and an ionisable drug (clomipramine) with two hydrophilic cyclodextrins: α-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin. The experimental study is performed via square wave voltammetry with 2PIs and 1PI solvent polymeric membranes and in both cases the electrochemical experiments enable the detection of inclusion complexes and the determination of the corresponding binding constant.

Voltammetry of the aqueous complexation-dissociation coupled to transfer (ACDT) mechanism with charged ligands

The study of the so-called aqueous complexation-dissociation coupled to transfer (ACDT) mechanism is extended to systems where the ligand species is not neutral and so the charge of the two transferable ions is different (z1 ≠ z2). This has a profound effect on the voltammetric response of the system, which shows a complex behaviour depending on the chemical kinetics, the difference between the lipophilicity of the two ions and the applied potential. Such response is modelled making use of the diffusive-kinetic steady state (dkss) approach, obtaining analytical expressions for the current-potential-time curves in normal pulse, derivative and differential multipulse voltammetries. In addition, manageable expressions for the concentration profiles, interfacial fluxes and interfacial concentrations of all the species either side of the liquid|liquid interface are derived. From them, the effect on the voltammograms of the characteristics of the chemical reaction and the lipophilicity of the ions is thoroughly studied, comparing the cases where the ions carry the same and a different charge. The last case shows some striking behaviours that can be understood from the analysis of the concentration profiles.

Transfer of complexed and dissociated ionic species at soft interfaces: a voltammetric study of chemical kinetic and diffusional effects

The ACDT mechanism is considered in which two different ionic species of the same charge can be transferred across a soft interface while they interconvert each other through a homogeneous chemical reaction.